1. Field of the Invention
The present invention relates to a process for fabricating a thin-film device such as a substrate, including thin-film transistors, of, for example, a liquid crystal panel. The present invention also relates to a thin-film device.
2. Description of the Related Art
In recent years, there have been proposed advanced liquid crystal panels having active matrix drives. A liquid crystal panel includes a liquid crystal filled between a pair of opposing substrates, one substrate being provided with a transparent common electrode and the other substrate being provided with a plurality of small pixel electrodes. The latter substrate includes pixel electrodes as well as gate bus lines, drain bus lines and thin-film transistors.
In producing the substrate including thin-film transistors, gate bus lines, gate electrodes and storage capacitor electrodes are formed on an insulating substrate, the gate bus lines are covered with an insulating layer, a semiconductor layer is formed thereon, and a channel protection film is formed thereon and is covered with an insulating layer. Thereafter, source electrodes, drain electrodes and drain bus lines are formed. Then, an insulating layer is formed thereon, and pixel electrodes are formed thereon. The insulating layer is perforated to connect the pixel electrodes to the source electrodes. In the substrate including thin-film transistors as described above, the gate bus lines, drain bus lines, thin-film transistors, pixel electrodes and the like are formed by laminating conducting layers and insulating layers.
The liquid crystal panel is required to have a high resolution and a high opening degree of apertures. For this purpose, it is requested that the gate bus lines, the gate electrodes and the like electrically connected to the gate bus lines be densely arranged; this in turn requires that the gate bus lines should be formed narrower and have reduced resistance. To maintain the high quality of the display, furthermore, storage capacitor electrodes are formed on the substrate using the same layer as the gate bus lines. A parasitic capacitance appears between a gate electrode and a source electrode which are overlapping. To maintain a high quality of the display the value of such a parasitic capacitance must be decreased. Also, high yields must be ensured while satisfying these requirements.
The liquid crystal panel is used not only as a display for information equipment but also used in a PDA, in a view finder, in a projector and in the like. The liquid crystal panels are relatively small in size, but it has been desired to provide a liquid crystal panel which is lighter in weight and has higher definition. In these liquid crystal panels, it has recently become necessary to employ low-temperature polycrystalline silicon thin-film transistors that can be formed integrally with the driver.
In order to satisfy both the requirements for narrower bus lines and for lower resistance, it has become necessary to decrease the width of the bus lines and to increase the thickness (or height) of the bus lines. When the thickness of the gate bus lines formed on the substrate is increased, however, the drain bus lines are sharply bent at positions where they are superposed on the gate bus lines when the drain bus lines are formed overlapped on the gate bus lines via an insulating layer. Therefore, the height of the drain bus lines changes at positions corresponding to the upper side edges of the gate bus lines and the etching residue remains; so the drain bus lines are apt to be cut or become defective.
In order to prevent the change in the height of the drain bus lines which may especially occur when the thickness of the gate bus lines is increased, or to prevent the etching residue from remaining unremoved, it is desired to incline the side surfaces of the gate bus lines with respect to the substrate, so that the upper side edges of the gate bus lines become smooth and the drain bus lines are gently bent.
When the gate bus lines are formed by isotropic etching which is normary adopted, however, the upper side edges of the gate bus lines do not become smooth. The inventors of the present case have discovered that the inclination of the side surfaces of all gate bus lines can be controlled to within a predetermined range of angle, by optimizing the baking temperature of the mask and the over-etching time. Upon inclining the side surfaces of the gate bus lines, the upper side edges of the gate bus lines become smooth, so the drain bus lines formed thereon are not broken, and dot defects can be eliminated.
In the etching condition by which the side surfaces of the gate bus lines are to be inclined, however, the angle of inclination of the side surfaces of the gate bus lines varies due to reaction gases generated during the etching, deteriorated etching solutions and variation in the temperature along the surface of the mask when baking the mask. The variation of the angle of inclination of the side surfaces of the gate bus lines can usually be tolerated, but some gate bus lines become too narrow or the angle of inclination is too small so that the side surfaces extend, in the shape of a hem of a skirt, along the substrate. The same also occurs for the gate electrodes and the storage capacitor electrodes formed together with the gate bus lines.
If the angle of inclination of the side surfaces of the gate bus lines is too small and the side surfaces are extended like the hems of skirts along the substrate, the areas of the gate bus lines located close to the substrate are increased and come into contact with other neighboring gate bus lines or gate electrodes, giving rise to the occurrence of short-circuiting in the same layer and causing the source electrodes, drain electrodes and gate electrodes to be overlapped on the gate bus lines to an excess degree, resulting in an increase in parasitic capacitances. In addition, when the channel protection film is to be formed by back-surface exposure using the gate, the shape of the channel protection film often becomes abnormal corresponding to the shape of the gate. As a result of investigation, it has been found that etching defects occur at the region where the gate bus lines, and gate electrodes and gate terminal-drawing portions electrically connected to the gate bus lines are densely arranged.
In the liquid crystal display device, furthermore, it is desired that the wiring is formed of, for example, aluminum or a metal material containing aluminum as a chief component in order to decrease the resistance of the bus lines. Such a metal material is layered on the glass substrate by, for example, sputtering and then patterned into a predetermined shape by etching or the like. However, unless the degree of vacuum is sufficiently high in the sputtering chamber prior to forming the films, aluminum or a metal containing aluminum as a chief component is apt to form bumps due to the subsequent thermal hysteresis, resulting in destruction of devices formed on the aluminum bus lines.
Moreover, a polycrystalline silicon thin-film transistor (p-SiTFT) has a mobility about 100 times as great as that of an amorphous silicon thin-film transistor (a-SiTFT), and makes it possible to form peripheral circuits and very small TFTs, which makes it possible to fabricate a liquid crystal panel that could not be achieved with the a-SITFTs. However, the p-SiTFT has a high ON current value and a high OFF current value, permitting a large current leakage. Therefore, dot defects may occur after the panel is fabricated, and the peripheral circuits formed using p-SiTFTs consume large amounts of electric power.
In order to reduce the OFF current value therefore, it has been proposed to form an offset around the gate using an LDD (lightly doped drain) structure. For example, the channel portion in the semiconductor layer is not doped with impurities but the portions on the outer side of the channel portion in the semiconductor layer are doped with impurities to form an HDD (heavily doped drain) to form a source electrode and a drain electrode. Here, small regions between the channel portions and source electrode/drain electrodes are doped with impurities to a smaller degree than in the portions of the source electrode and the drain electrode, to thereby form the LDD, i.e., to form an offset.
Japanese Unexamined Patent Publication (Kokai) No. 7-235680 discloses a method for fabricating a thin-film transistor in which an offset is formed. This method includes the steps of forming a semiconductor layer on an insulating substrate, forming a gate electrode having a broad bottom surface (having inclined side surfaces) on the semiconductor layer, doping the semiconductor layer with impurities using the gate electrode as a mask, and etching the inclined side surfaces. A thick portion of the gate electrode does not permit impurities to pass therethrough but the inclined side surfaces of the gate electrode permit impurities to pass therethrough a little. Therefore, the portions covered by the inclined side surfaces of the gate electrode in the semiconductor layer form the LDD, i.e., the offset. According to this prior art, however, the gate electrode must be formed of a material that permits impurities to pass therethrough. According to the production method of this prior art, therefore, limitation is imposed on the materials that can be used as the gate electrodes; i.e., it is not allowed to use aluminum or the like metal which is most adapted to form gate electrodes and gate bus lines. Besides, impurities may pass through the thick portion of the gate electrode, deteriorating the performance of the channel.
The object of the present invention is to solve the above-mentioned problems and to provide a process for fabricating a thin-film device having a plurality of bus lines of a smooth shape and connection portions electrically connected to the bus lines and a thin-film device fabricated by such a process.
Another object of the present invention is to provide a process for fabricating a thin-film device, by which the formation of bumps on the surfaces of the bus lines can be prevented and the devices formed on the bus lines can be prevented from being destroyed.
A further object of the present invention is to provide a process for fabricating a thin-film device, which is capable of suitably forming an LDD structure, and to provide a thin-film device fabricated thereby.
A process for fabricating a thin-film device according to the present invention comprises the steps of forming a conducting layer that can be anodically oxidized on a substrate; etching said conducting layer to form a plurality of bus lines having upper surfaces parallel to said substrate and inclined side surfaces and connection portions electrically connected to said bus lines and having upper surfaces parallel to said substrate and inclined side surfaces; and anodically oxidizing said bus lines and said connection portions, so that said bus lines and said connection portions include inner conducting portions and outer insulating oxide films covering said inner conducting portions, respectively.
In this process, the bus lines are gate bus lines formed in the substrate of a liquid crystal panel, for example, and the connection portions electrically connected thereto are gate electrodes. These bus lines and connection portions have upper surfaces parallel to the substrate and inclined side surfaces. This makes it possible to satisfy both requirements of reducing the width of the bus lines and increasing the thickness of the bus lines.
When such bus lines and connection portions are formed by etching, the inclination of the side surfaces of the bus lines and of the connection portions may vary and a portion of some bus lines and connection portions located close to the substrate is extended in the shape of a hem of a skirt along the substrate so that the area of the portion close to the substrate may become greater than a predetermined area. Due to the anodic oxidation, however, the upper portions of the bus lines and of the connection portions turn into outer insulating oxide films. Therefore, even when the bus line and the connection portion are extended like a skirt by etching, the skirt portion is the outer oxide film, and no short-circuiting takes place between the gate bus lines and the neighboring conductors.
Preferably, the etching step is carried out so that the side surfaces of said bus lines and the side surfaces of said connection portions are inclined at angles within the range from 20 degrees to 60 degrees, on average, with respect to said substrate. More preferably, the etching step is carried out so that the side surfaces of said bus lines and the side surfaces of said connection portions are inclined at angles within the range from 30 degrees to 50 degrees, on average, with respect to said substrate.
Preferably, the process further comprises the step for forming a mask on said conducting layer prior to said etching step, and the step for ashing said substrate including said mask between said mask-forming step and said etching step. The process further comprises a step for forming a mask on said conducting layer and a step for baking said mask prior to said etching step, wherein the temperature for baking said mask in said baking step is so set that said mask will have a relatively small rigidity so that an outer portion of said mask is pushed away from said conducting layer due to a reaction gas in said etching step. In this case, the temperature for baking said mask in said baking step is not higher than 115xc2x0 C.
Moreover, the etching step is carried out so that the side surfaces of said bus lines and the side surfaces of said connection portions are outwardly convex. Furthermore, the etching step is carried out so that the angles between the upper surfaces and the side surfaces of said bus lines and of said connection portions are obtuse angles.
The process further comprises an ionic milling step for removing part of the outer oxide films to expose the inner conducting portions after said step of anodic oxidation.
Moreover, a thin-film device according to the invention comprises at least a substrate, a plurality of bus lines provided on said substrate, and connection portions electrically connected to said bus lines, said bus lines and said connection portions being formed of an anodically oxidizable metal and having upper surfaces parallel to said substrate and inclined side surfaces, respectively, said bus lines and said connection portions including inner conducting portions and outer insulating oxide portions formed by anodic oxidation to cover said inner conducting portions, respectively.
The thin-film device has the same actions and effects as those of the above-mentioned device.
Preferably, said thin-film device is a substrate including thin-film transistors. In this case, the substrate including said thin-film transistors is a substrate of a liquid crystal display device, said bus lines are gate bus lines, and said connection portions are gate electrodes of said thin-film transistors, said thin-film device further comprising an insulating layer covering said bus lines and said connection portions, a plurality of drain bus lines arranged on said insulating layer to cross said gate bus lines, and a plurality of pixel electrodes. The device further comprises storage capacitor electrodes arranged on said substrate and made of the same material as said gate bus lines and said connection portions. Or, said thin-film device is an MIM diode.
Preferably, said anodically oxidizable metal comprises at least the one selected from the group consisting of Al, Ta, Alxe2x80x94Si, Alxe2x80x94Ta, Alxe2x80x94Zr, Alxe2x80x94Nd, Alxe2x80x94Pd, Alxe2x80x94W, Alxe2x80x94Ti, Alxe2x80x94Tixe2x80x94B, Alxe2x80x94Sc, Alxe2x80x94Y, Alxe2x80x94Pt, and Alxe2x80x94Pa.
Preferably, the side surfaces of said bus lines and the side surfaces of said connection portions are inclined at angles within the range from 20 degrees to 60 degrees, on average, with respect to said substrate. More preferably, side surfaces of said bus lines and the side surfaces of said connection portions are inclined at angles within the range from 30 degrees to 50 degrees, on average, with respect to said substrate.
Preferably, the side surfaces of said bus lines and the side surfaces of said connection portions are outwardly convex. Furthermore, the angles between the upper surfaces and the side surfaces of said bus lines and of said connection portions are obtuse angles.
Besides, at least two outer oxide films of said plurality of bus lines and said connection portions contact each other and said contacting outer oxide films electrically isolate the inner conducting portions covered by said contacting outer oxide films.
A conducting portion separate from said bus lines and said connecting portions is arranged close to said bus lines or said connecting portions, said separate conducting portion includes an inner conducting portion and an outer insulating oxide portion covering said inner conducting portion, the outer oxide film of said separate conducting portion contacts at least one outer oxide film of said bus lines and of said connection portions, and said contacting outer oxide films electrically isolate the inner conducting portions that are covered by said contacting outer oxide films.
According to another feature of the present invention, a process for fabricating a thin-film device comprises: forming a conducting layer composed of an anodically oxidizable metal on a substrate; etching said conducting layer in a predetermined shape; forming a second oxide film on said conducting layer by anodic oxidation after a first oxide film with a predetermined thickness is formed on said conducting layer; and washing said substrate, whereby said first oxide film is removed by said washing, and said second oxide film is not removed by said washing but remains on said conducting layer so as to cover said conducting layer.
According to this process for production, the conducting layer composed of an anodically oxidizable metal forms gate electrodes and gate bus lines. The first oxide film and the second oxide film being laminated thereon are formed on the conducting layer. The second oxide film is formed under the first oxide film by anodically oxidizing the metal that forms the conducting layer. The first oxide film is located on the surface of the second oxide film. The first oxide film is a crystalline oxide film affected by the anodic oxidation and can be easily removed upon washing the substrate. The second oxide film is not removed by washing but remains on the conducting layer so as to cover the conducting layer. Since the first oxide film can be removed by washing, particles that adhere to the surface of the conducting layer can be removed together with the first oxide film. Accordingly, no bumps form on the surfaces of the bus lines, and devices formed on the bus lines are prevented from being destroyed. The second oxide film is not removed but remains, and is effective in reducing the width of the bus lines and in increasing the thickness of the bus lines as in the anodically oxidized film of the first invention.
In this case too, the anodically oxidizable metal includes at least one of Al, Ta, Alxe2x80x94Si, Alxe2x80x94Ta, Alxe2x80x94Zr, Alxe2x80x94Nd, Alxe2x80x94Pd, Alxe2x80x94W, Alxe2x80x94Ti, Alxe2x80x94Tixe2x80x94B, Alxe2x80x94Sc, Alxe2x80x94Y, Alxe2x80x94Pt, and Alxe2x80x94Pa.
Preferably, the first oxide film is one of a naturally oxidized film or a hydrated film formed on the surface of said anodically oxidizable metal. Preferably, the first oxide film has a thickness from 50 nm to 100 nm. Preferably, the washing step is executed using ultrasonic waves of not lower than 200 KHz.
Preferably, the thin-film device is a substrate including thin-film transistors. In this case, the process further comprises a step for forming an insulating film on said substrate and a step for forming a semiconductor layer on said substrate after the second oxide film has been formed, wherein the step for etching said conducting layer forms gate electrodes and gate wirings. Or, the process further comprises a step for forming a semiconductor layer on said substrate and a step for forming an insulating film on said substrate prior to forming said conducting layer, wherein the step for etching said conducting layer forms gate electrodes and gate wirings.
Preferably, the step for etching said conducting layer forms a gate electrode having an upper surface parallel to said substrate and a tilted side surface.
According to a still further feature of the present invention, a process for fabricating a thin-film device comprises the steps of forming a semiconductor layer having a predetermined shape on a substrate; forming an insulating film on said substrate to cover said semiconductor layer; forming a conducting layer composed of an anodically oxidizable metal on said substrate in such a shape as to cover a portion of said semiconductor layer and to form gate electrodes having upper surfaces parallel to said substrate and inclined side surfaces; anodically oxidizing said gate electrodes; forming said insulating film into a predetermined shape using said gate electrode including the anodically oxidized film as a mask; and injecting impurities into said semiconductor layer using said gate electrodes including said anodically oxidized film and said insulating film as a mask to form an offset in said semiconductor layer.
This feature makes it possible to produce a substrate including polycrystalline silicon thin-film transistors having an offset formed by the LDD structure.
Moreover, the present invention provides a thin-film device comprising a substrate, a semiconductor layer formed in a predetermined shape on said substrate, an insulating film covering a portion of said semiconductor layer, a gate electrode formed on said insulating film, and an anodically oxidized film of said gate electrode formed on said insulating film so as to cover said gate electrode, said anodically oxidized film having a shape as viewed from above which is identical to the shape of said insulating film as viewed from above and having an annular portion in innular contact with said insulating film about said gate electrode, a portion of said semiconductor layer located on the outer side of said insulating film forming a source electrode and a drain electrode, and a portion of said semiconductor layer covered by said annular portion of said anodically oxidized film forming an offset on the inner side of said insulating film.
Moreover, the present invention provides a liquid crystal display device comprising a first substrate comprising the above-described thin-film device having a plurality of thin-film transistors, a second substrate opposed to the first substrate, and a liquid crystal layer filled between the first and second substrates.